Many high end microwave assemblies are still hermetically packaged, such as transmit and receive modules for phased arrays, components for defense applications, power amplifiers, assemblies requiring chip and wire construction, high performance devices and circuit operating in the upper microwave and mm-wave bands, and so on. The reasons include ensuring reliability under environmental variations and a lack of organic or polymer protective layers that do not reduce or interfere with device performance due to factors such as dielectric loss or attenuation, or due to changing the electrical impedance of transmission lines contained in or on the device. Hermetic packaging has substantial drawbacks. The substantial cost and yield impact may be attributed to the specialized nature of the parts used in hermetic packaging, such as metal or ceramic housings, solderable or weldable lids, hermetic seals such as glass-metal seals for connectors, and the manual labor usually required for assembly and test and rework. Meanwhile, most consumer electronics traditionally operating at lower frequencies have been able to move to lower cost non-hermetic packaging through the use of protective coatings, underfill polymers, encapsulants and the like. Such approaches enable more automated batch production on large area circuit boards. Non-hermetic cavity packaging has been done in some cases; however, in environments where there is high humidity and fluctuating temperatures, condensation of water can occur inside the package. In addition, ionic contaminates such as sodium, potassium, and calcium can come from environmental sources including fingerprints, the air or salt water can penetrate many non-filled cavity structures and produce electrical reliability problems such as conductivity between circuits and/or corrosion. The problems from such condensation and ionic contaminates can be eliminated by employing polymer or silicone encapsulations over the electronics components.
Traditional approaches used to package devices for consumer electronics do not work well on microwave devices and the circuit boards on which they are mounted because of the field interaction from the transmission lines in the circuits extend into the surrounding mediums and often extend into the encapsulants or coatings producing problems such as attenuation, changing transmission line impedance, and otherwise interfering with the function of the circuit. For example, a microstrip transmission line with a coupled-line filter can have substantial field lines that interact on or around the signal line upper surfaces. The same is also true of a coplanar waveguide (CPW) transmission line. Traditional materials such as those based on silicones, acrylates, and even high performance vapor deposited coatings such as parylene have substantially higher than air dielectric constants changing the transmission line impedance or function. Moreover, the loss tangents of these materials can substantially alter circuit performance in ways that cause it to deviate from design targets, particularly at high microwave frequencies. While such materials can be valuable in non-hermetic packaging, their use in microwave devices and modules without some means of separating them from the microwave devices will usually result in detrimental interactions. To address these problems, some microwave devices such as monolithic microwave integrated circuits (MMICs) and circuit boards have been designed with buried transmission lines to minimize field interactions with the surrounding environment, specifically with, for example, underfill materials or encapsulants. While this addresses the problem of the surrounding environment substantially changing their function, it does not enable optimal performance as the losses in many semiconductors and circuit board materials are high and typically increase with frequency. As frequency moves up from 2 to 10 to 40 GHz or above the problem of losses in packaging materials becomes increasingly demanding. Accordingly, there remains a need to have a low cost non-hermetic packaging technology for devices containing microwave circuitry with transmission lines for example such as microstrip or CPW or any primarily air-dielectric or suspended transmission lines, including waveguides and air-dielectric coaxial transmission lines, that allows RF, microwave, and mm-wave components, devices, and assemblies containing such transmission lines to operate with minimal interaction with the dielectrics surrounding them while protecting them from the environment.
A possible solution to this problem, as further disclosed herein, is the use of a very low-k layer of material, such as a foam, that does not substantially interfere with the operation of a circuit designed for operation in air or in a vacuum environment. Such a layer can be applied thick enough to minimize field interactions, for example 0.5 to 2 mm or more thick, and can be used as a “spacer layer” to an outer protective set of layers. Exceptionally low-k materials, such as expanded urethane foams, sol-gels, aerogels, porogen filled polymers, and syntactic foams with low dielectric constants, for example, below approximately two, can serve as a spacer layer without adversely affecting all but the most sensitive devices (such as, for example, high Q or narrow band pass filters). Unfortunately, such low-k foams as currently exist are usually also porous and permeable to moisture and ionic contaminant penetration. A solution to this is to seal the low-k material using one or more sealing layers. U.S. Pat. No. 6,713,867 B2 to Mannak et al. discloses a syntactic foam protected by a “moisture proof top layer” identified as a “polymer lacquer”. U.S. Pat. No. 6,713,867 B2 does not, however, identify any candidate materials to satisfy this requirement and does not identify the need for an ionic barrier or more than one sealing layer. It also does not identify the importance of choosing low ionic contaminate materials or materials based their ability to resist contaminants such as sodium, potassium, and other ionic conductors with high mobility. There are many polymer lacquers that would not work well and many polymer lacquers that would allow moisture to penetrate. For example, RTV silicone contains ionic contaminants. Polyacrylic acid, polyvinyl alcohol, polyvinyl pyrolidone, phenolic resins such as novalaks or anything with a hydroxy group would be poor choices due to their ionic mobility.
In addition, the reference does not identify various improvements to the art to enable its practical use. Such improvements include, for example, the use of adhesion layers between the silver paints and ionic barriers such as, for example, electronic grade silicones; the need to protect conductive paint, for example silver-filled conductive paints which can be used for EMI shielding, from corrosion; the use of EMI absorbing layers such as, for example, graphite filled silicones; or the use of secondary protection layers such as ALD coatings on circuits. Thus, despite the fact that syntactic foams have been available since the 1960's and other low-k materials, such as aerogels, have been available to the packaging industry for over a decade, these materials have still not found use for microwave device packaging with the exception of spacer layers in antenna construction.i Thus, many high performance microwave devices for critical applications in airborne and marine environments, such as T/R modules, radar modules, chip and wire assemblies, still require expensive hermetic packages.
U.S. Pat. No. 6,423,566 B1 to Feger et al. discloses a method to protect the interconnect layers of a semiconductor chip or wafer where the electrical interconnect layers of the device are contained in the dielectric, said dielectric including materials that include a low-k dielectric. Disclosed are a number of possible thin polymeric barriers that can be applied to the wafer or chip after manufacturing it to protect the exposed portions of dielectric materials in the interconnect layers from ionic contamination and moisture ingress. While U.S. Pat. No. 6,423,566 B1 discloses a polymeric barrier to protect a dielectric disposed on an interconnect structure that may include a low-k material, it does not teach a packaging method or technology or structure for circuits or assemblies or the use or addition of low-k materials applied to a device at a thickness, for example typically 500 to 700 microns, sufficient to prevent deleterious field interactions at a distance from the device's RF or microwave transmission lines to prevent subsequent layers from interfering with the device performance. In the reference, the low-k material was formed as part of an electrical interconnect structure that totals on the order of 5-15 microns in thickness and is instead formed as part of the integrated circuit manufacturing process and is therefore not applied as a component of a device packaging technology. Furthermore, it does not teach the application to microwave devices or microwave device packaging. It does not teach the use of multiple layers for EMI blocking or attenuation. In addition, the ionic and moisture barrier layers taught in the reference are less than one micron thick whereas the ionic and moisture barrier layers required for RF packaging are typically on the order of 100-500 times thicker. Finally, while a RF or microwave device is not taught, if the semiconductor device in U.S. Pat. No. 6,423,566 B1 was, in fact, a microwave device and it was being packaged on a circuit board, it would still require the solution for non-hermetic packaging taught herein, i.e., providing thick low-k layer, ionic sealing layers and EMI blocking layers or absorbing layers as no provisions are made to solve EMI coming from the device itself or from a circuit to which it is attached.